Frequency control system



E. F. GRANT FREQUENCY CONTROL SYSTEM Aug. 17, 1954 2 Sheets-Sheet 1 Filed July 20, 1945 Fig.|.

Amplifier 147 Fectifier hq Duscnmmutor .Generator I ALMS Fig.8.

INVENTOR Eugene F. Grunt.

BWM ATTORNEY E. F. GRANT 2,686,875 FREQUENCY CONTROL SYSTEM Aug. 17, 1954 Filed July 20, 1945 2 Sheets-Sheet 2 0. Voltage 0 Voltage 0 Frequency Voltage Plate Voltogc Grid Voltage 0 Frequoncy g 5 lst Detector 2 nd Detector 08 Loss Video I25 Fi Indicator INVENTOR Eugene F.Grom.

ATTORNEY about the predetermined Patented Aug; 17, 1954 FREQUENCY CONTROL SYSTEM Eugene F. Grant, Swissvale, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application July 20, 1945, Serial No. 606,204

This invention relates to frequency control sys tems and it has particular relation to systems for maintaining substantially constant the output frequency of electrical oscillation generators.

In accordance withthe invention, a system is provided wherein the frequency of the output generator is forced to oscillate within a small range about a predetermined frequency. This predetermined frequency may be substantially constant and may be determined by the resonant frequency of a tuned circuit, such as a cavity resonator. Such a system not only has a rapid response but is of relatively simple construction. Furthermore, the rate of oscillationof the frequency of the oscillation generator frequency may be made substantially greater than any hunting frequency, which may be present in the system: Under such circumstances, special precautions for eliminating hunting are unnecessary. In addition, the system permits the utilization of a tuned or resonant circuit, such as a cavity resonator, having a relatively low Q.

A system designed in accordance with the invention may be employed generally as a source of constant frequency. As a more specific example, a system embodying the invention may be employed as a local oscillator in a superheterodyne receiver for receiving brief, intermittent code signals from distant radio transmitters. Since the deviation in the frequency of an oscillation generator controlled in accordance with the invention from a predetermined constant frequency may be maintained readily within one part in 100,000, the system when employed as a local oscillator in a superheterodyne receiver per mits the utilization of a relatively narrow pass band for the intermediate frequency stages of the receiver.

According to a further aspect of the invention, the local oscillator of a superheterodyne receiver is modulated in frequency. Such modulation desirably modifies the effectivepass characteristics of the intermediate frequency stages of the superheterodyne receiver. y

It is, therefore, an object of the invention to provide an improved frequency control system.

It is a further object of the invention to provide an improved system for controlling an oscillation generator wherein the frequency output of the generator is periodically varied within a small range about 6 Claims. (Cl. 250-36) is continuously oscillated about the resonant freput of the generator within a small range quency of a tuned circuit.

It is another object of the invention to provide an improved frequency control system having means for normally maintaining the frequency output of an oscillation generator within a predetermined small range and having normally ineffective means which is effective on deviation of the output of the oscillation generator from the aforesaid range of restoring the frequency of the oscillation generator to such range.

It is a further object of the invention to modify the effective pass band of the intermediate frequency stages of superheterodyne receivers.

It is also an object of the invention to frequency modulate the output of the local oscillator of a superheterodyne receiver.

Other objects of the invention will be apparent from the following description taken in conjunction with the accompanying drawing, in which: igure 1 is a schematic view with parts incross section of a frequency control system embodyi ing the invention} i Fig. 2 is a graphical representation illustrating the characteristics of tuned circuits;

Fig. 3 is a graphical representation illustrating the performance of a phase discriminator and a control circuit employed in a system embodying the invention;

Fig. 4 is a graphical representation illustrating the performance of a system illustrated in Fig. l

Fig. 5 is a graphical representation of the firing characteristic of a gaseous discharge tube;

Fig. 6 shows in block form a superheterodyne receiver;

Fig. 7 is a graphical representation showing the pass characteristic of an intermediate frequency 1 stage of a superheterodyne receiver; and

section showing a Fig. 3 is a schematic View with parts in cross modified form of the invention. Referring to the drawings, Figure 1 shows a system which includes an oscillation generator I, a tuned circuit 3, a phase discriminator 5, an amplifier La control circuit 9, a search circuit ll, an alternating current generator I 3, and a power supply source I 5. In order to facilitate an understanding of the invention, the operation of the system will be briefly described. This will be followed by a more detailed discussion of the components of the system.

The oscillation generator I may be of any type having a frequency-control element and which is directly responsive to modulated signals applied thereto. For the purpose of discussion, it

will be assumed that the oscillation generator I is a velocity-modulated tube of the type commonly referred to as a reflex Klystron. This tube has a grounded cathode electrode la and a reflector electrode lb. As well understood in the art, the frequency output of the oscillation generator l is a function of the voltage applied between the electrodes la and lb.

The voltage applied between the electrodes la and lb in the oscillation generator is the resultant voltage across a capacitor I! and a resistor IS. The voltage across the capacitor ll determines the average direct voltage applied between the electrodes la and lb. The resistor I9 is connected across the terminals of the generator 13 for the purpose of developing thereacross a small alternating voltage. This alternating voltage serves to frequency modulate the output of the oscillation generator I.

Let it be assumed that the capacitor H has applied thereto a substantial charge of such polarity as to make the terminal A negative with respectto ground and that this charge is slowly dissipated through resistors connected across the plates of the capacitor. The voltage between the terminal A and ground is such as to produce a frequency output of the oscillation generator I which is above the resonant frequency of the tuned circuit 3, which is illustrated as a transmission-type cavity resonator. As the charge on the capacitor I1 is dissipated, the frequency output of the oscillation generator I decreases until it reaches the resonant frequency of the cavity resonator 3. A portion of the output of the oscillation generator I is supplied to the input of the cavity resonator 3 and the output of the cavity resonator is rectified in any suitable manner as by a crystal rectifier or detector l8. As the frequency of the output of the oscillation generator I passes through the resonant frequency of the cavity resonator, the output of the crystal rectifier l8 reverses in phase.

It will be recalled that a frequency modulation is applied to the oscillation generator by the alternating voltage drop across the resistor 19, which is connected across the alternating-current generator [3. This frequency modulation appears at the output of the crystal rectifier 18 as an alternating voltage which is of either two opposed phases depending on which side of the resonant frequency of the cavity resonator 3 the oscillation generator I is operating.

The phase discriminator is responsive to the phase of the output of the crystal rectifier IS. The output of the phase discriminator, if of sufficient amplitude, may be applied directly to the control circuit. However, in the specific system illustrated in Fig. l, the amplifier l is employed for inverting and amplifying the output of the phase discriminator 5 to a value suitable for operating the control circuit.

The purpose of the control circuit 5 is to lower the voltage of the terminal A to a substantially lower value when the frequency of the oscillation generator 1 passes through the resonant frequency of the cavity resonator 3. It will be recalled that as the capacitor ll discharges, the voltage of the terminal A becomes less negative with respect to the ground and consequently the frequency of the oscillation generator I decreases. When the frequency of the oscillation generator i passes through the resonant frequency of the cavity resonator 3, the output of the crystal rectifier l3 reverses in phase and the resulting output of the phase discriminator 5 when inverted and amplified by the amplifier 1 becomes sufficient to actuate the control circuit 9. In response to such actuation, the control circuit restores the charge on the capacitor l'i and consequently increases the frequency of the oscillation generator i to a value above the resonant frequency of the cavity resonator 3.

After the control circuit has operated, the capacitor l1 again starts to discharge. and the entire cycle is repeated. The frequency of repetition of the cycle of discharge and charge may have any desired value. In practice, a frequency of repetition of 200 times per second has been found suitable. The circuit is readily capable of maintaining the voltage across the capacitor ll within lus or minus 0.05 volt of a predetermined value.

As previously pointed out, a charge is. placed in the capacitor ll having a polarity selected to make the terminal A negative with respect to ground. This charge may be applied to the capacitor I! in any desired manner as by a manual charging operation. However, in the specific embodiment of Fig. l, the search circuit H is provided for properly charging the capacitor ll whenever the capacitor requires charging as, for example, when the system of Fig. 1 is initially placed in operation.

Turning now to a more detailed description of the system illustrated in Fig. l, the oscillation generator l, in addition to its cathodeelectrode la and its reflector electrode ib, includes a cavity resonator lc which is maintained at a positive voltage relative to the cathode in any suitable manner, as by means of a battery Id. The output of the cavity resonator lc is obtained from a conventional output loop lc which is coupled to the input of the cavity resonator 3 by means of a coaxial cable 2 l The cavity resonator 3 is of the transmission type and may include an adjustment unit 23. By manipulation of the adjustment unit, the resonant frequency of the cavity may be adjusted. The output of the cavity resonator 3 is rectified by means of the crystal rectifier or detector I8 and applied across a resistor 25. The crystal rectifier l8 may be of any desired construction such as the silicon detector commonly employed in the art. Velocity-modulated tubes such as the oscillation generator l, cavity resonators similar to the cavity resonator 3, and silicon detectors suitable for the rectifier it are Well known in the art as shown, for example, by reference to Ultrahigh Frequency Techniques by J. G. Brainerd ct al., 1942, published by D. Van Nostrand Company, Inc., New York city, and Klystron Technical Manual, 1944, published by the Sperry Gyroscope Company, Inc, Brooklyn, N. Y.

Since it is important that the cavity resonator 3 accurately maintain a predetermined resonant frequency, the cavity resonator should be designed to operate with great stability. If desired, for example, the cavity resonator 3 may be located in a container which is maintained at a constant temperature and constant pressure.

Since the operation of the system is dependent to a considerable extent on the characteristics of a tuned circuit, such as a cavity resonator, a brief discussion of such characteristics is given at this time. If a constant voltageinput is supplied to the cavity resonator 3, and the frequency of the input is varied through the resonant fre quency of the cavity resonator, a curve similar to the curve B of Fig. 2 is obtained. For this curve abscissae represents frequency and ordionant frequency; her [8 decreases the capacitor sented by the curve D represented bythe curve C2 the cavity resonator 3, the voltage it across the respect to each other.

tionship is obtained for all cases wherein the rate of .change of the nates represent voltage. outputfrom the crystal rectifier l8. t

It will be observed that. thegreatest response is obtained for an input havinga frequency equal to the resonant frequencyfo of the cavity reso-y nator. For frequencies on either side of the resthe output of the crystal rectiappreciably in amplitude.

If the input to the cavity resonator 3 1s frequency modulated, an alternating voltage is obtained from the crystal rectifier I 8 having a phase relationship relative to the frequencymodulated input which=is dependent on the direction of deviation of the center frequency of the frequency-modulated input fromthe reso nant frequency of the cavity resonator. Forexample, let it be assumed that thevoltage across I'l is adjusted to produce a frefromthe oscillation generator I than the resonant frequency in quency output which is lower of the resonator, and represented by the dotted line f1 in Fig. 2. Let it be assumed further that the alternating voltage acrossthe resistor I9 in.

Fig. 1 produces a frequency modulation of the output of the oscillation generator I which is represented by the curve C in Fig. 2. For this curve C, abscissae represent frequencies and ordinates represent time.

If the frequency-modulated output of the oscillation generator i represented by thecurve C is passed through the cavity resonator 3 of Fig. l,

resistor 25 may be reprethe voltage across the of Fig. 2. For this curve, abscissae represent time and ordinates represent voltage amplitude.

Let it be assumed next that the voltage across the capacitor I1 is adjusted to produce a frequencyoutput f2 from the oscillationgenerator I which is above the resonant frequency in of the cavity resonator 3. The frequency-modulated output of the oscillation generatorl then may e E2 is shown be represented by the curve C2 of Fig. 2. The

curve C2 is;plotted on thesame coordinates employed for the curve i ,When the output of the oscillation; generator is passed through resistor 25 is similar to that represented bythe curve D2 in Fig. 2. The coordinates for the curve D2 are similar to those employed forj jthe curve D. i

The principal distinction b'etweenthe curves D and D2 is that they are in phase opposition with This opposed phase rela center frequencies 1 and of the curves Oand C2 are on opposite sides of the resonant frequency it.

frequency and ordinates represent the slope of the curve B or the derivative of the function represented by the curve B with respect'to frequency. The change in sign of the curve Eas.

it passes through the resonantfrequency represents the change in phase relationship-between thecurves D and D2. It will be noted that the curve E is greatest as it passes through the resonant frequency ft. 9 Corrrent generator i3.

sequent1y,-a control circuit may be made extremely sensitive to changes in the curve E for frequency changes adjacent the frequency ft.

The rate of change of the curve E as it passes through the resonant frequency may be varied byvarying the frequency modulation of the oscillation generator. As the deviation of the oscillation generator I produced by frequency modulation increases, the slope of the curve E as it passes through the resonant frequency decrease a substantial slope usually is desirable.

Although a control circuit could be designed which responds tothe changein sign as the curve E passes through the frequency in, it is convenient to raise the entire curve E entirely above the zero voltage line as shown inFig. 3. In Fig. 3, a curve which corresponds to the curve E of Fig. 2 except that-the curve E2 is represented entirely above the zero voltage axis by a distance represented by the voltage E3. It is the purpose of the phasediscriminator 5 and the amplifier I of Fig. 1 to convert the voltage drop across the resistor 25 which is represented by the curve E in Fig. 2 to a quantity corresponding to that represented by the curve E2 in Fig. 3. To this end, the-voltage drop across the resistor 25 is algebraically added to a constant alternating voltage in the phase discriminator; Referring toFig. 1, the phase discriminator 5 includes a tetrode Zlhaving a plate electrode 21a, a screen-grid electrode 21b, a control-gridelectrode 21c and a cathode electrode 27d. The plate electrode 27a is connected through. a plate resistor 29 and conductors 3| and 33 to thepositive terminal lta on the power supply source l5.

This source l 5 may be of any suitable construction, but as illustrated it comprises a voltage divider i5b havin a central terminal I50 which is grounded. The outer terminals l5a and I 511 of the voltage divider are connected respectively to-the positive and negative terminals ofa battery [6. Consequently, voltages derived from the voltage divider between the terminals |5a and will be positive with respect to ground. Voltages derived between the terminals I50 and I 5d of the voltage divider will be negative with respect The voltage drop across the resistor 25 is coupled to the tetrode 27 through a coupling ca-, pacitor and a grid resistor 43. Consequently, the input fromthe resistor 25 appears acros the grid resistor 43 b'etweenthecathode and the con trol-grid electrode 2'50 of the tetrode 2'5.

An additional voltage is introduced into the circuit connecting the control-grid and cathode electrodes of the tube 27 by connecting the oath ode resistor 39 through suitable resistors 45 and All across the terminals of the alternating-cur} The frequency of the output of the alternating-current generator I 3 may have various suitable values. If the system of Fig. 1 isto be employed in aircraft, the alternatingcurrent generator is may be designed to produce a frequency of 800 cycles per second, and may be employed for supplying energy toother equipment on the aircraft. It will be recalled that the I3 also is employed ground through a cathode re for frequency modulating the oscillation generator l.

The parameters of the system are so selected that the voltage drop across the resistor 39 as a result of current supplied by the alternating-current generator i3 is in phase with the voltage drop across the grid resistor 43 when the center frequency of the oscillation generator 1 is below the resonant frequency of the cavity resonator 3. It follows that when the center frequency of the oscillation generator I is above the resonant frequency of the cavity resonator 3, the voltage drops across the resistors esand 43 are in phase opposition. Consequently, the rectified output of the cavity resonator 3 and the alternating voltage of constant amplitude supplied by the alternatingcurrent generator l3 are algebraically added in the input circuit of the tetrode 21. This corresponds to adding algebraically the voltages represented by the curve E of Fig. 2' to the alternating voltage of constant amplitude represented by the value E3 in Fig. 3 in order to produce the resultant represented by the curve E2 in Fig. 3.

It should be noted from Fig. 3 that values of E2 to the right of the resonant frequency in are always lower in amplitude than the voltage E3. Values of the curve E2 for frequencies lower than the resonant frequency in are always higher than the voltage E3. Consequently, the point G of intersection of the curve E2 with the voltage E3 is determined accurately by the resonant frequency of the cavity resonator 3.

The tetrode 2'5 inverts and amplifies a voltage corresponding to that represented by the curve E2 of Fig. 3. The output of the phase discriminator 5 is coupled through a coupling capacitor 49 to a tetrode 5| which serves to invert and further amplify the output of the phase discriminator. The tetrode 5| includes a plate electrode 51a, a screen-grid electrode 5H), a control-grid electrode 5lc and a cathode electrode 5ld. The plate electrode cm is connected through a plate resistor 53 and the conductor 33 to the terminal l5a of the power supply source. The screen-grid electrode 511) is connected to a centrally disposed tap on a voltage divider formed by resistors 55 and 51. These resistors are connected in series between the conductor 33 and ground. The control-grid electrode 5la is connected to ground through a grid resistor 59 and is connected to the coupling capacitor 49. A cathode resistor Bl connects the cathode electrode Bid to ground.

Assuming that the values of ordinates are properly selected, the output of the amplifier I is represented by the curve E2 in Fig. 3. This output is coupled to the control circuit 9 through a coupling capacitor 62.

The control circuit 9 essentially is a relaxation oscillator employing a gaseous discharge tetrode 63 which conveniently may be of the type commonly referred to as a thyratron. The tetrode 63 has a plate electrode 53a, a shieldgrid electrode 63b, a control-grid electrode 63c and a cathode electrode 63d. The plate electrode 63a is connected through a conductor 65 and a resistor 61 to a terminal 156 on the voltage divider which is positive with respect to ground. The

shield grid electrode 63b and the cathode electrode 63d are connected to a terminal I51 on the voltage divider which is negative with respect to ground. A capacitor 69 and a current limiting resistor H are connected in series between the plate electrode 63a and the cathode electrode 63d.

In order to bias the control grid electrode 630 negatively with respect to the cathode electrode 63d, the control-grid electrode is connected through a resistor 13 to an adjustable tap 15 on a potentiometer 11. The potentiometer is connected between the points l5d and i5 on the voltage divider. The control-grid electrode 530 also is connected to the coupling capacitor 62. It will be noted that the terminal A is connected to the plate electrode 63a through a resistor 19.

The bias on the control grid electrode 630 is adjusted by manipulation of the tap 15 to a value such that a discharge of the tetrode 63 will be initiated when the voltage represented by the curve E2 in Fig. 3 has a value above that represented by the dotted line 8!. This value is such that when the frequency of the oscillation generator I decreases from a value above the resonant frequency in of the cavity resonator 3, a discharge will be initiated in the tetrode 53 immediately after the frequency of the oscillation generator I passes through the resonant frequency of the cavity resonator 3.

When this occurs, the capacitor 5% which has previously been charged from the power supply sourve l5 discharges through the tetrode 63. Furthermore, the terminal A is connected through the resistor 19 and the plate-cathode discharge path of the tetrode 63 to the terminal I51 of the power supply source. Since the terminal l5f represents a substantial negative voltage relative to ground and since the drop through the tetrode 63 is small, this connection of the terminal A tends to restore the charge on the capacitor IT. The recharging of the capacitor l'l continues until the tetrode 53 stops conducting and the capacitor 69 has been recharged to a value such that the plate electrode 63a is less negative with respect to ground than the terminal A. The capacitor ll then resumes its discharge through the circuits across its terminals. This entire cycle is repeated at a frequency which may be of the order of 200 times per second and may be adjusted to maintain the charge on the capacitor 11 within a very small range. It will be recalled that the charge on the capacitor I! in turn, determines the center frequency of the output of the oscillation generator l.

As previously pointed out, the search circuit l l is essentially a relaxation oscillator. It employs a gaseous discharge tetrode 83 which may be similar to the tetrode 63. The tetrode 83 includes a plate electrode 83a, a shield-grid electrode 83b, a control-grid electrode 330 and a cathode electrode 83d. The plate electrode 83a is connected to the terminal 15 of the voltage divider 15b through a circuit which includes, in series, a conductor 85, resistors 81, 89 and 9|, and a conductor 93. The resistor 9! is shunted by a fixed resistor and an adjustable resistor 91. The

plate electrode 83a also is connected to the terminal A through a resistor 99. The cathode 83d and the shield-grid electrode 83b are connected to ground through a cathode resistor Mil which is shunted by a capacitor I03. A resistor its also connects the cathode to the junction between the resistors 95 and 91.

In order to understand the operation of the search circuit, it should be noted that the oathode electrode 8311 is maintained at a substantial negative voltage with respect to ground which is determined by the drop across the cathode resistor 10!. This resistor is connected to the terminal [5f of the voltage divider through the tetrode 83 resistor I and the parallel ocombination of resistors SI, 95 and 91. l

The terminal A is centrally disposed on a loop circuit which may be traced from the terminal [5e of the voltage divider tors 61 and 1.9, the terminal A, resistors and 89 and the 99, 81 parallel combination of resistors 9| 95 and 91 to terminal I51 on the voltage divider. The parameters of this loop circuit may be so selected that the terminal A and the adjacent plate electrode 8311, when. the system is initially energized, are substantially more positive than the cathode electrode83d with respect to ground. Under these conditions, a discharge isinitiated in the tetrode 83a. Since the voltage drop between the plate and cathode electrodes of the tetrode83 is small, the terminal A is brought close to the voltage of the cathode electrode 83d with respect to ground and the capacitor l1 receives a charge which tends to maintain the terminal A negative with respect to ground.

After the capacitor has been charged, the discharge in the tetrode 83 ceases andthe capacitor I! then begins to discharge through the circuits connecting its terminals. The rate of discharge of the capacitor 11 is controlled by thetime constant of the capacitor and its associated discharge circuits. If the discharge continues for such a time that the plate electrode 83a of the becomes substantially positive with respect to the cathode electrode 83d for the grid connections employed, the tetrode 83 again discharges and restores the charge of the capacitor l1. However, it the charge on the capacitor I1 is maintained within close limits by the operation of the control circuit 9, no further operation of the tetrode 83 occurs.

By inspection of the search circuit, it will be observed that the control-grid electrode 830 is connected to a tap on a voltage divider which, in turn, is connected between the plate and cathode electrodes of the tetrode 83. This voltage divider is formed by the resistors 81, 39, the parallel combination of resistors SI, 95 and 91 and the resistor IE5. Consequently, as the plate electrode 83a becomes more positive with respect to the cathode electrode 83d,the control-grid electrode 830 also becomes more positive with respect to the cathode electrode. The advantages of such a connection of the control-grid electrode may be understood by reference to Fig. 5.

In Fig. 5 ordinates represent plate-to-cathode voltage and abscissae represent grid-to-cathode voltage of a tube. The. curve H represents the firing characteristics of a gaseous-discharge tube suitable for the tetrode. 83. Such a tube fires when the plate and grid voltages are such as to fall within theshaded area of Fig. 5.

Let it be assumed that the tube normally has a negative bias represented by the grid voltage J. Asthe plate voltagebecomes more positive with respect to the cathode electrode, the control-grid voltage also becomes more positive. Consequently, as the plate voltage becomes more positive, the relationship between grid and plate voltages relative to the cathode follow a curve similar to that depicted by the curve JK. When thesevoltages reach the curve H, the tube fires.

If the grid voltage were held constant as the plate voltage increased, the line JK would appear in Fig. 5 as a vertical line. Such a line would intercept the curve H at a very small angle and slight variations in grid or plate voltages would. result in a substantial variation in the firing |5b through the resis initiated in the tetrode 83.

10 point of the tube. By forcing the grid voltage to become more positive as the plate voltage becomes more positive, the line JK intercepts the curve H at a substantial angle and the point at which the tube fires consequently does not vary appreciably for small variations in grid or plate voltages.

The operation of the system as a Whole now will be considered. When the system is initially energized, the cathode electrode 836! of the tetrode 83 is at a substantial negative voltage relativ to the plate electrode 93a. Consequently, .a discharge is initiated inthe tetrode 83. This eiiectively connects the terminal A to the oathode elecrode 83d through the resistor 99, which has a relatively low value, and through the lowresistance path between the plate and cathode electrodes of the tetrode. For this reason, the terminal A is lowered to a substantial negative voltage relative to. ground and the capacitor ll charges in such a direction as to tend to maintain the terminal A at this voltage.

. After the capacitor ll has been charged, the discharge in the tetrode 83 terminates and the capacitor ll begins to discharge through a first path including the resistors 79 and 61 and a second path including the resistors 99, 8?, 89 and associated resistors. Since the resistance of the first path is relatively smaller in value, the rate of discharge of the capacitor ll may be controlled effectively by suitable selection of the value of the resistor 79. l

Iithe capacitor ll discharges sufiiciently to bring the voltage of the plate electrode 830. to a substantial positive value relative to that of the cathod electrode 8365, a second discharge is This cycle is repeated to produce a saw-tooth characteristic represented in Fig. 4 by the dotted curve L. In Fig. 4 abscissae represent time and ordinates represent voltage. This .curve represents the voltage of the terminal A relative to ground produced by operation of the search circuit l I. v

. Itwill be noted that when the system is first energized, the tetrode 83 of Fig. 1 rapidly lowers the voltage of the terminal A to a substantial negative value relative to ground which is represented in Fig. 4 by the reference character M. As the capacitor ll discharges, the voltage of the terminal A rises towards a value N. If the voltage reaches the value N, the conditions are proper for initiating a second discharge in the tetrode 83 which again lowers the voltage of the terminal A to the value M.

The voltage M is applied to the reflector electrode of the oscillation generator I (Fig. 1) and has a negative value relative to ground of sufficient magnitude to produce a frequency output from the oscillation generator I which is above the resonant frequency ]o of the cavityresonator 3. In addition, the reflector electrode lb of the oscillation generator has an alternating voltage applied thereto which corresponds to the voltage drop across the resistor i9. However, since the output frequency of the oscillation generator is above the resonant frequency of the cavityresonator '3, the voltage output from the crystal rectifier l8 establishes a voltage across the grid resistor 43 which is in phase opposition to the voltage drop across the resistor 39. It will be recalled that the voltage drop across the resistor 39 is produced by current supplied from the alternating-current generator 13. Since these volt- -ages are in phase opposition, theoutput of the amplifier l is insuflicient to initiate a discharge of the control-circuit tetrode 63.

As the voltage of the terminal A becomes less negative along the line MN of Fig. 4, it reaches a value P at which the output frequency of the oscillation generator equals the resonant frequency of the cavity resonator 3. When the output frequency passes through this point, the output of the crystal rectifier l8 reverses in phase and the voltage drops across the resistors 39 and 43 are in phase. The voltage output of the amplifier 1, corresponding to the voltage E2 of Fig. 3, consequently reaches a value above the bias voltage represented by the dotted line 8| in Fig. 3 and initiates a discharge of the control-circuit tetrode I53.

When the system of Fig. 1 is initially energized, a charge is stored in the capacitor 69 which maintains the plate electrode 63a positive with respect to its cathode 63d. The plate electrode 53a is maintained initially at a substantial positive voltage relative to ground which is represented in Fig. 4 by a curve RS.

When the discharge is initiated in the tetrode t3, the capacitor 69 discharges through the tetrode. Furthermore, the plate electrode 63a is connected to the cathode electrode 53d through a low-resistance path. Since the cathode 63d. is maintained at a substantial negative voltage relative to ground, the plate electrode 63a also is brought to a substantial negative voltage relative to ground. Inasmuch as the terminal A is connected to the plate electrode 63a, the discharge of the tetrode 63 tends to restore the charge of the capacitor ll. The charge of the capacitor I! is represented in Fig. 4 by the curve PT.

By reference to Fig. 4, it Will be observed that the voltage of the plate electrode 63a relative to ground at a time corresponding to the point P drops rapidly to a substantial negative value relative to ground which is represented by the point U. At the point U, the discharge in the tetrode 63 terminates and the capacitor 69 begins to charge. As the capacitor 69 charges, the voltage of the plate electrode 63a relative to ground increases along the curve UV. When this voltage becomes less negative than the terminal A relative to ground, the charging of the capacitor i1 terminates and this capacitor then begins to discharge through the paths connecting its terminals, as represented by the curve TW. The time available for charge of the capacitor H along the curve PT may be controlled over an adequate range by suitable selection of the time constant of the capacitor 69 and its associated charge and discharge circuit.

When the voltage of the terminal A relative to ground reaches the value W, the output frequency of the oscillation generator i again passes through the resonant frequency of the cavity resonator 3, and through the detector, phase discriminator and amplifier initiates a second discharge of the tetrode 63. The capacitor 69 again discharges through the tetrode 63 and the voltage of the plate electrode 63a drops to a low negative value relative to ground, as represented by the curve VX in Fig. 4. This, in turn, restores the charge on the capacitor l1, and the cycle is repeated at any desired rate such as 200 times per second. By operation of the control circuit, it is possible to maintain the voltage across the capacitor l1 within a range of plus or minus 0.05 volt. This, in turn, maintains the output frequency of the oscillation generator lv within a small range such as a range or one part in 100,000.

It should he noted that as long as the control circuit maintains the charge on the capacitor if, the terminal A never reaches the voltage represented by the point N in Fig. 4, and the search circuit remains inoperative. However, should the control circuit. fail to operate at any point, such as the point Y in Fig. 4, the voltage represented by the curve L would increase to a value N ca.-

pable of initiating a discharge in the searchcircuit tetrode 83. This would restore the volt.-

age of the terminal A to. a value M corresponding to the initial value M. The operation or the circuit then would be similar to that explained for the initial energization thereof.

It should be noted that the voltage drop across, the resistor l9 introduces at all times a ripple in the voltage of the terminal A relative to ground. This ripple is indicated for part only oi Fig. 4 by a curve Z.

The phase discriminator and amplifier of Fig. 1, may employ type SAKS tubes. in conventional amplifier circuits. The principal distinction introduced in these circuits by the; invention is the connection of the cathode resistor 39 across; the alternating-current generator i3; Since these circuits. otherwise are of substantially conventional construction, it is believed that further description thereof is. unnecessary.

However, it may be of help to list. typical values for the components of the control and B2=l0,000 micro-microfarads 69=10,000. micro-microiarads l'l-=0.5 microfarad [03:1 microfarad The resistors have the following values:

13=470,000 ohms Tl=100,000 ohms 'Il =1600 ohms. 61==470,000 ohms 19:2.2 megohms [9:10 ohms 99:1600 ohms 81:4.7 megohms 89:1 megohm 9I=l megohm =75,000 ohms 9"l=100-,000 ohms [05:8200 ohms l0l=100,000 ohms For such a circuit the voltage divider I511 may be designed to provide positive voltages from the terminal [5a and I5e which are, respectively, 150 and volts relative to ground. The voltage divider also provides from terminals 15d and I5 negative voltages which are respectively 325. and 255 volts relative to ground.

It will'be noted that several adjustable controls are provided forthe system of Fig; 1. The voltage M (Fig. 4) may beadjusted over a suitablera-nge by manipulation of the adjustable tap associated with the resistor 91. This does not affiect-appre ciably thetotalavoltagesweep of the search circuit which is represented by the distance NM ini-Fig. 4

This voltage sweepmay-beofi theorderioi 50 volts;

adjustable type, the resonant frequency thereof may be adjusted by manipulation of the adjust.- ment unit 23. 1

. The voltage drop across the resistor I9 produced by the alternating current generator I3 may be of the order of 0.1 to lvclt.

Various applications of. the system illustrated in Fig. 1 may be made. A portion of the output of the oscillation generator I i may be obtained through a suitable connection I00. This connection may be employed as a source of constant frequency for an desired purpose. For example, the system of Fig. 1 may be employed as a local oscillator in an otherwise conventionalsuperhet-v erodyne receiver. in Fig. 6. I

Referring to Fig. 6, an antenna I01 is illustrated for receiving code signals having a carrier frequency which may be of theorder of 10,000 megacycles. These signals are supplied to a first detec tor which also receives energy from a local oscillator I09. The local oscillator may be of the type illustrated in Fig. 1. It will be noted that the connection I06 of Fig. 1 is illustrated in Fig. 6. i In the first detector, the local oscillations are mixed with the signal and the resulting intermediate frequency is supplied to one or more intermediate frequency stages III. These intermediate frequency stages may be of any conventional structure suitable for operation at a suitable intermediate frequency. For operation with intermediate frequencies of 30 or 60 megacycles, the

Such a receiver is illustrated intermediate frequency stages may have a band pass width of the order of 0.5 to 5 megacycles. In equipment actually constructed a band pass width of 2.6 megacycles was employed. I As illustrated in Fig. 6, such a stage may employ a coupling capacitor I I3 and an inductor II5 for coupling two tubes II! and H9. The capacitor II3, together with the inductor II 5 and any inductance and capacitance inherent in the-connections between the tubes, are tunded to resonate at the intermediate frequency.

. The output of the intermediate frequency stages is suppliedto a second conventional detector I2 I. The output of thesecond conventional detector, in turn, is amplified in a suitable video amplifier I23 and translated in any suitable indicator device I25, such as a cathode-ray-tube indicator device. 1

Certain benefits are obtained by frequency modulating the output of the local oscillator I09. These may be understood more readily by reference to Fig. 7 which shows a family of curves plotted on coordinates wherein abscissae represent frequency and ordinates represent loss in intermediate frequency stages of a superheterodyne receiver. If a single frequency is passed through the intermediate frequency stages, a curve similar to the curve I21 is obtained. This curve shows substantially no loss at the resonant frequency In of the intermediate frequency stages, but loss increases appreciably as the frequency departs from this resonant frequency. Consequently, the curve I 27 represents very selective performance of the intermediate stages. The curve I21 applies to frequencies which either increase or decrease from the resonant frequency.

If a small frequency modulation is incorporated in the local oscillations, a curve similar to the curve I29 is obtained. By inspection of Fig. 7, it will be noted that the curve I 29 shows increased loss adjacent the resonant frequency of the intermediate frequency stages but it showsdecreased losses for frequencies displaced appreciably from the resonant frequency in. Consequently, the curve I29 shows a Wider band pass characteristic. As the frequency modulation increases in magnitude, band pass characteristics similar to those represented by the curves I 3| and I33 are obtained. Consequently, it will be noted that by introducing frequency modulation in the local oscillations, it is possible to widen the band pass frequency range of an intermediate frequency amplifier. It will be recalled that frequency modulation of the oscillation generator I is produced by the alternating voltage drop across the resistor I9 (Fig. l).

Fig. 8 shows a modification of the system illustrated in Fig. 1. In Fig. 8, the oscillation generator I is coupled to a cavity resonator 3 which corresponds to the cavity resonator 3 of Fig. 1. However, the cavity resonator 3' has a resonant frequency which may be varied by reciprocation of a rod I35. The rod is connected to anelectrode I 3'! mounted in the cavity resonator. The output from the cavity resonator 3' is obtained from the crystal rectifier I8 and is supplied to the phase discriminator 5 and the amplifier I in a manner similar to that illustrated in Fig. 1. It will be noted that the generator I3 again is connected to the phase discriminator 5 in the manner discussed with reference to Fig. 1.

The generator I 3 also is employed for actuating the rod I35 in order to vary the resonant frequency of the cavity resonator 3'. To this end, the generator is connected through a halfwave rectifier I39 to a solenoid I4I which is employed, when energized, for attracting the magnetic armature I43. When the solenoid MI is energized, it moves the rod I35 downwardly, as viewed in Fig. 8, against the resistance of a spring I45. When the solenoid is deenergized, the spring I45 restores the rod I35 and. the associated electrode I31 to their initial positions. This continued oscillation of the electrode I31 results in an oscillation of the resonant frequency of the cavity resonator 3 about an average resonant frequency.

Referring to Fig. 2, it will be noted that variation of the resonant frequency of a cavity resonator has the effect of introducing an alternating component into the output of the crystal rectifier I9. For example, if the frequency ii of Fig. 2 is applied to a cavity resonator and the resonant frequency of the cavity is oscillated within predetermined limits, a voltage similar to that represented by the curve D will be obtained from the crystal rectifier I8. On the other hand, if a frequency i2 is supplied. to the cavity resonator, a curve similar to curve D2 will be obtained from the crystal rectifier. Consequently, the output from the. amplifier 1 of Fig. 8 will correspond to the curve E2 of Fig. 3. This output may be rectified by a rectifier I41 and filtered by a filiter I49 to produce a direct voltage which is substantially free from ripple. The direct voltage is applied between the reflector electrode and cathode electrode of the oscillation generator I of Fig. 8.

Let it be assumed that the oscillation generator I of Fig. 8 requires a voltage between its reflector and cathode electrodes similar to that represented by the voltage E3 of Fig. 3 in orderto produce normally the desired frequency f0. This voltage normally is produced by the amplifier I of Fig.' 8 and is rectified and filtered by the rectifier I4! and filter I49 prior to application to the tion generator 1 increases above the value in, the output of the amplifier I would drop as clearly shown by reference to the curve E2 of Fi 3. The decrease in voltage output of the amplifier 1 would result in a decrease in magnitude of the voltage applied between the reflector and cathode electrodes of the oscillation generator I.

On the other hand, if the frequency of the oscillation generator i should tend to decrease below the frequency in, the output of the amplifier 1 would tend to increase as clearly shown by reference to curve E2 of Fig. 3. This voltage output of the amplifier is rectified, filtered and ap plied to the oscillation generator I, and tends to increase the frequency produced by the oscillation generator. It will be. understood that the voltage represented by the curve E2 of Fig. 3 is rectified, filtered and applied to the oscillation generator in such polarity as to make the reflector electrode negative relative to the associated cathode electrode. From a consideration of this discussion, it will be observed that the system of Fig. 8 tends to maintain a constant frequency from the oscillation generator without recourse to the control and search circuits of Fig. 1.

Although the invention has been described with reference to certain specific embodiments thereof, numerous modifications are possible. Therefore, the invention is to be defined only by the appended claims as interpreted in view of the prior art.

I claim as my invention:

, 1. A frequency controlling system comprising an oscillator, a low loss resonant tank circuit fed with Waves derived from said oscillator, said tank circuit being tuned to a desired frequency of operation of said oscillator, means for frequency modulating said oscillator with frequency con-v trolling waves, an amplitude modulation detector for detecting wave energy appearing in said tank circuit, a phase detector supplied with waves from said first mentioned detector and with waves having a frequency corresponding to said control frequency, and instrumentalities responsive to the output of said phase detector for adjusting the mean frequency of operation of said oscillator.

2. A frequency controlling system comprising an oscillator, a low loss sharply resonant tank circuit, resonant at the desired frequency of operation of the oscillator, fed with waves derived from said oscillator, said tank circuit being tuned to a desired frequency of operation, means for frequency modulating said oscillator with frequency controlling waves, the frequency swing being such as to' vary the frequency of the oscillator from one side of the characteristic curve of said resonant circuit to the other, an amplitude modulation detector for detecting wave energy appearing in said tank circuit, a phase detector suppliedwith waves from said first mentioned detector and with waves having a frequency correspond ing to said control frequency, and instrumentalities responsive to the output of said phase d'etec tor for adjusting the mean frequency of operation of said oscillator.

3. A frequency controlling system comprising, in combination, an ultra-high-frequency oscillator, a resonant cavity having input and output terminals, said cavity being tuned to a predetermined desired frequency of operation of said oscillator, means for supplying oscillations from said oscillator to said input terminals, means for frequency-modulating said oscillations in response to relatively low frequency control waves, the amplitude of said control waves being suflicient to vary the frequency of said oscillations over a range extending on either side of said desired frequency, means for detecting relatively low frequency amplitude modulation waves thereby produced in the envelope of said oscillations at said output terminals, means for combining said control frequency waves and modulation waves algebraically to produce a resultant wave, and means for adjusting th mean frequency of said oscillator in response to an amplitude characteristic of said resultant wave.

4. The combination specified in claim 1 in which said oscillator is given a center frequency which is varied from a value displaced from the resonant frequency of the resonant circuit through the resonant frequency of said resonant circuit.

5. The combination specified in claim 4 in which said instrumentalities reverse the direction of variation of said center frequency substantially in response to the passage of said center frequency through said resonant frequency.

6. The combination specified in claim 1 in which said frequency modulation comprises periodic variation about a center frequency at a control frequency, and variation of said center frequency from a value displaced from said desired frequency .through said desired frequency, and in which said instrumentalities include a switching tube having an input electrode on which a voltage of said control frequency is impressed and an output circuit which is made conductive by said voltage to reverse the direction of variation of said center frequency.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,104,801 Hansell Jan. 11, 1938 2,173,180 Peterson Sept. 19, 1939 2,245,685 Koch June 17, 1941 2,250,102 Klemperer July 22, 1941 2,287,925 White June 30, 1942 2,404,568 Dow July23, 1946 2,428,265 Crosby Sept. 30, 1947 2,434,293 Stearns Jan. 13, 1948 2,462,857 Ginzton et al Mar. 1, 1949 2,475,074 Bradley July 5, 1949 

